Experimental Demonstration of the Collisionless Plasmoid Instability below the Ion Kinetic Scale during Magnetic Reconnection

J. Olson, J. Egedal, S. Greess, R. Myers, M. Clark, D. Endrizzi, K. Flanagan, J. Milhone, E. Peterson, J. Wallace, D. Weisberg, and C. B. Forest

Phys. Rev. Lett. 116, 255001 – Published 20 June 2016

Abstract

The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas.

Received 28 February 2016

DOI:https://doi.org/10.1103/PhysRevLett.116.255001

Physics Subject Headings (PhySH)

Laboratory studies of space & astrophysical plasmas Magnetic reconnection

Plasma Physics

Authors & Affiliations

J. Olson, J. Egedal, S. Greess, R. Myers, M. Clark, D. Endrizzi, K. Flanagan, J. Milhone, E. Peterson, J. Wallace, D. Weisberg, and C. B. Forest

Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, Wisconsin 53706, USA

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Vol. 116, Iss. 25 — 24 June 2016

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Images

Figure 1
Theoretical phase diagram indicating different regimes of reconnection where multiple $X$ lines separated by plasmoids are expected for both collisional and collisionless reconnection as a function of the Lundquist number and the normalized system size. The shaded regions map out accessible plasma parameters of both TREX and MRX. Other notable results from tearing experiments are marked with red ovals. The results presented here fall into the collisionless regime where multiple $X$ lines are not expected from theory.
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Figure 2
(a) Schematic of the Wisconsin Plasma Astrophysics Laboratory in the TREX configuration, including the magnetic diagnostic suite. To drive reconnection, the internal coils are pulsed opposite the steady state field provided by the external Helmholtz coil. (b) Calculated vacuum magnetic fields during a pulse overlaid with an illustration of the plasma inflows and outflows as the reconnection current layer propagates toward the center of the vessel. The shaded gray box indicates the areal coverage of the array of 144 magnetic flux probes.
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Figure 3
Profiles of (a) the reconnecting magnetic field $B_{Z}$ and (b) the out-of-plane current density $J_{ϕ}$ measured by the flux array. The contour lines of constant magnetic flux $Ψ$ show the in-plane projection of magnetic field lines. (c) The out-of-plane Hall magnetic field $B_{ϕ}$ measured by the linear magnetic array. (d) Profiles of the electron density and temperature measured by a multiple Langmuir probe array. The temporal axis in (c) and (d) is converted to a spatial axis knowing the velocity of the current layer, $V_{R} \sim - 4 km / s$ , as it flows passed each probe. (e) Profiles at $Z = 0 m$ of $J_{ϕ}$ and $B_{Z}$ inferred from (a) and (b).
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Figure 4
(a)–(f) Measured 2D profiles of the out-of-plane current density $J_{ϕ}$ and magnetic flux contours; $Ψ = const$ . The occurrence of separate plasmoids is clearly visible for $t = 128 μ s$ and $t = 134 μ s$ . The dashed lines indicate the midplane where $B_{Z} = 0$ . Here $1 μ s \times f_{c e} \sim 50$ , where the electron gyrofrequency $f_{c e}$ is evaluated with the average reconnection magnetic field. (g),(h) Time evolution of $J_{ϕ}$ and the inductive electric field $E_{ϕ}$ evaluated at the midplane as a function of $Z$ and $t$ . The dashed lines correspond to the midplanes from frames (a)–(f). The $O$ and $X$ points are marked with red and black lines, respectively. (i) Magnetic flux $Ψ$ observed at the midplane for $t = 134 μ s$ . The $X$ points are located as local maxima, while the $O$ points coincide with the local minimum. $Δ Ψ$ provides a measure for the size of the islands. (j) Statistical study of the size of the magnetic islands. Islands from 85 discharges are binned according to their size measured by $Δ Ψ$ of panel (i). The data are consistent with both exponential and power-law dependencies for the island occurrence rate as a function of the island size.
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